Kindly mention all details. What is the status of the sales order line and delivery line? There are various scenarios
a) Unreserve
b)If already pick released
c)Data Fix scenario
d)fiddling with Move order allocation

Apologies for the delay. I am down with a chest infection. Anyway here is the solution.
Reduce the unwanted quantity by back ordering on the sales order line ie reducing the quantity.
Complete the shipment of the partially picked ones and interface the original quantity. This will result in a new sales order line which you cancel from sales order.
Ramji

If you partially picked sales order which contains 10 quantity and now you want to make reserve of 5 items from this sale order then ship the sales order with quantity as 5 and then in back order field automatically 5 quantity will appeared. Then ship it. By this that sales order will again come with another line of 5 quantity as awaiting shipping. By this we can cancel that line to reserve items.

I am having a concern, could you please help out me. How to un-reserve the item qty for a sales orders. These orders are all old orders, we have to un-reserve the qty. I have tried to back order them and have updated the shipment qty as zero and save them. Observed that back order qty has updated with the reserved qty. the below are status of the orders